De Blasio said Clinton has championed issues such as income equality and paid family leave, policies that have played prominently in the Democratic primary.

The Democratic mayor dismissed concerns over Clinton's tendency to "flip-flop," and argued that it "took some real courage" for the former secretary of state to come out against the president's trade agreement after calling it the "gold standard."

"She has said what she believes in. This is a very sharp progressive platform," de Blasio said. "She has the ability to follow through on it."